Control Strategy for Starter Generator in UAV with Micro Jet Engine

Transcription

1 Control Strategy for Starter Generator in UAV with Micro Jet Engine Jun-ichi Itoh, Kazuki Kawamura, Hiroyuki Koshikizawa and Kazuyuki Abe Department of Electrical, Electronics and Information Engineering, Nagaoka University of Technology, Niigata, Japan Development Department, YSEC Co., Ltd, Niigata, Japan Abstract This paper proposes control strategy of a starter generator connected to a jet engine for an unmanned aerial vehicle system. Thrust is generated by both the jet engine and propellers which are powered by the jet engine through the starter generator. A flight range can be extended since energy density of the jet engine in the developed system is higher than battery energy density in the conventional system. Moreover, the starter generator directly connects to the jet engine and rotates at high speed for miniaturization. The proposed control strategy achieves the starting, the powering and the cooling operations with the starter generator. It is confirm through an experiment of a -kw prototype, that the prototype system achieves the maximum conversion efficiency of 9.7%. The minimum generator current THD is 6.5% at 7 r/min. Further, the exhaust nozzle temperature is controlled within the maximum deviation of % regarding to the command value in study state. Keywords Starter generator, Jet engine, Unmanned aerial vehicle(uav), V/f control. I. INTRODUCTION Recently, unmanned aerial vehicles (UAVs) have been actively studied for rescue activities in disaster [ ]. In particular, the multicopter-type UAV has two advantages. First, it is easy to approach danger zones because of unmanned operation. Second, the multicopter-type UAV does not need a designated landing space. However, the multicopter-type is generally powered by batteries []. The flight range and carrying weight are limited because of the battery energy density [5]. Therefore, UAV with a jet engine has been developed [6]. In the developed UAV system, thrust is generated by both the jet engine and propellers which are powered by the jet engine through the starter generator. The flight range can be extended since the energy density of the jet engine is higher than the battery energy density. Furthermore, the developed UAV system is also be used as an emergency power supply owing to the starter generator. An auxiliary power unit (APU) is generally used for starting and cooling the jet engine [7 8]. However, the use of APU leads to the increase in cost and size of the system. Furthermore, the rotation speed of the generator in APU is low because the generator is connected to the jet engine through reduction gears [9 ]. Therefore, the generator tend to be large in a high power capacity system. In this paper, the UAV system with a jet engine and the control strategy of the starter generator are proposed. In the developed UAV system, only the starter generator is used for starting and cooling, which eliminates the use of APU. Furthermore, the starter generator connects directly to the jet engine and rotates at high speed for miniaturization. The challenge of this paper is the achievement of the stable operation through the proposed control strategy even when the starter generator transits among operation modes, i.e., starting mode, powering mode, and cooling mode without APU and reduction gears. In particular, the synchronous frequency command limiter and the output power limiter are used in the proposed control method. In addition, modulation method is modified by the estimated intersection phase based on synchronous PWM. Through the experiments, it is confirmed that the prototype achieves the maximum conversion efficiency of 9.7%, the minimum generator current THD of 6.5% at 7 r/min. Further, the exhaust nozzle temperature is controlled within the maximum deviation of % compared to the command value in the steady state. II. DEVELOPED UAV SYSTEM Figure shows the configuration of the developed an UAV system. The jet engine and the starter generator are directly connected without reduction gears. The jet engine powers six propellers through the starter generator. In the aerial applications, weight reduction of the starter generator is required from the viewpoint of flight range. Thus, the starter generator is rotated at high speed for miniaturization and weight reduction. Figure shows the mode transition diagram of the developed UAV system. A host controller selects the operation mode. The operation modes are described as follows; A. All Off Mode This mode is a stationary state. The power converter is not operated(gate off). B. Standby Mode The DC/DC converter boosts the DC-link voltage from the battery voltage to V.

2 C. Startup Mode The starter generator is driven by the AC/DC converter in order to assist both the ignition of the jet engine and the acceleration up to 5 r/min. The host controller controls the starter generator speed in this state. Further, the jet engine controls exhaust nozzle temperature. D. Run Mode The powering operation is performed in the range of the rotation speed from 5 to 7 r/min, and the battery is charged. The jet engine controls the speed, whereas the starter generator controls the output power. E. Stop Mode The engine output is halted, whereas the starter generator decelerates and cools the jet engine. When the exhaust nozzle temperature of the jet engine is cooled to 5 C or less, the operation of the power converter is stopped. Then, the operation mode is shifted to the all off mode. III. MODULATION METHOD FOR EVEN-ORDER HARMONIC COMPONENTS SUPPRESSION Jet engine G Starter generator Power converter AC/DC DC/DC Battery DC/AC DC/AC Fig.. Configuration of developed UAV system. Start (A) All Off Mode Shift Command (B) Standby Mode Shift Command (C) Startup Mode Shift Command Rotation Speed >5 r/min (D) Run Mode Shift Command (E) Stop Mode M 6 M Propeller A. Continuous PWM The starter generator is driven at the rotational speed of the jet engine, the carrier frequency, and the fundamental frequency are close to each other. This leads to the loworder harmonic components and the beat components on generator current. Thus, in the Run Mode, the nine-pulse synchronous PWM technique is used. Figure shows the voltage command with a modulation index of.8 and a triangular carrier with a frequency ratio of nine when the continuous PWM is applied. As shown Fig. (a), the voltage command v u is compared with the triangular carrier to generate a PWM signal in general. As shown Fig. (b), the modulation index command V m and the red carrier u mc are compared to generate a PWM signal. The deformed carrier u mc is calculated by um umc (,8 ) (), sin where is the phase of inverter voltage command and u m is the triangular carrier. The proposed modulation method estimates the intersection phase of the modulation signal and the carrier using a look-up table of the deformed carriers in the software, then outputs a voltage command according to the estimated intersection phase. It can be implemented in a micro-controller because this proposed modulation method is implemented without changing the hardware. Table I shows the estimated intersection phase patterns of the continuous PWM. As shown in Table I, the estimated intersection phases of sectors zero and nine are determined to be and 8 because of the synchronous PWM, respectively. The relationship between the phase and the modulation index command of the deformed carrier u mc of the sectors,,, and is tabulated as Rotation speed DC-link voltage Injection Port Temperature <5 C (a) Flowchart of developed UAV system operation. Mode A 5 V Mode B V Mode C Powering operation Acceleration assist Ignition 5 r/min assist Time Mode D Mode E Cooling operation Mode A (b) Overview operation of jet generator. Fig.. Mode transition diagram of developed UAV system. look-up table,,, and. te that,, and are the estimated intersection phases referred from these look-up tables. For the sector 5 and later, it is not necessary to prepare the tables in order to estimate the phase using the symmetry each 9. The proposed modulation method acquires the estimated intersection phase in each sector using the phase referred to from look-up table to and the relationship in Table I, and outputs a voltage command according to the estimated intersection phase. The generated PWM signal is equivalent to that generated by analog control. Therefore, even-order harmonic components do not occur in PWM signal. B. Discontinuous PWM Figure shows a modulation signal and a carrier with frequency ratio of 9 and a modulation index of.8 when

3 (a) (b) - Number of sector u mc um sin v V sin Fig.. Waveforms of voltage commands and carriers with continuous PWM. Sector TABLE I Proposed estimated phase patterns of continuous PWM. Look Up Table Phase of Intersection Point m m u m V m Sector 9 Look Up Phase of Table Intersection Point X :Phase by Look Up Table X the discontinuous PWM is employed. The discontinuous PWM signal v xd shown in Fig. (a) is calculated by adding the following offset to the three phase modulation signal v x. v v v, x u, v, w xd x offset v offset v if v v, max max min v if v v, min min max v v v v max and v v v v min max[,, ] u v w min[,, ] u v w (). As mentioned in Section A, a deformed carrier u md in Fig. (b) is used. The intersection phases in sectors and are defined as and 9 in advance, respectively. Therefore, the deformed carriers of sectors, and are only required to estimate the intersection phases. The deformed carrier u md is calculated by u md u m sin sin( ) ( 6 ) (). te that in the section of more than 6 in sector, the intersection phase is set to 6 when the modulation index is.577 or less. Table II shows the estimated intersection phase patterns of the discontinuous PWM. The relationship between the phase and the modulation index command of the deformed carrier u md of the sector,, and is defined as in look-up table,, and. By referring the phases,, using the modulation index from these look-up tables, the intersection phase of the carrier and the modulation signal in each sector is estimated by the relationship shown in Table II. By using this method, the symmetry of the PWM signal is secured even in the discontinuous modulation, and the even-order harmonic (a) (b) Jet engine V dc - ー V dc Number of sector G Starter generator PI Voltage control V m {sin sin( )} iu Inverter f sw= khz or 9 pulse Vdc Interleaved Converter ia ib ic f sw=6 khz Fig. 5. Configuration of power converter. V V dc bt u md Duty command calculate a i a ー i a PI v a V dc Duty command calculate b Duty command calculate c Duty command calculate d d a d b d c d d id Carrier Phase shifted carrier Fig. 6. Control block diagram of DC/DC interleaved converter. V m V m u m {sin sin( )} Fig.. Waveforms of voltage commands and carriers with discontinuous PWM. TABLE II Proposed estimated phase patterns of discontinuous PWM. Sector Look Up Table Phase of Intersection Point Sector Look Up Table Phase of Intersection Point X :Phase by Look Up Table X components do not occur in the PWM signal. IV. ibt Modulation Modulation Vbt Battery Gate signal S a, b Gate signal S c, d CONTROL STRATEGY FOR STARTER GENERATOR Figure 5 shows the configuration of the power converter. This converter consists of a three-phase inverter and a four-leg interleave DC/DC converter. Since the battery voltage is approximately 5 V, the DC/DC converter is required to boost the voltage to V in order to drive the inverter. Figure 6 shows the control block diagram of DC/DC interleaved converter. The DC link voltage V dc is regulated to the command value. The current imbalance among four legs is suppressed by the current control of

4 Exhaust nozzle temperature control T g - T i a i b i c i d I P P T Output power command depend on g P out i bt Power generation output control - V bt P out PI Saturarion M Rate Limit UX Saturarion mode Output power limiter - I g each leg. Moreover, the carrier of each two legs is phase shifted by half a period compared to the other two legs. As a result, the switching frequency is equivalently doubled and the current ripple is reduced to half []. Figure 7 shows the control block diagram of the three phase inverter. The power control is operated in the run mode, whereas the V/f control is employed in the other modes. The rotation speed of the jet engine is suddenly reduced because the output power of the jet engine is not sufficiently high when the inverter control switched from the V/f control to the output power control or exhaust nozzle temperature control for the run mode. In other words, the self-sustained operation of the jet engine is difficult at the low speed. In order to solve this problem, a synchronous frequency command limiter is applied. As a result, the speed is kept constant until the jet engine output becomes sufficiently high. During the startup mode, the generator torque suddenly changes. In order to prevent overcurrent in this operation, an output power limiter is introduced. Consequently, the synchronous frequency command is compensated in order to avoid the sudden change in the torque. Furthermore, around the rated speed, discontinuous PWM is employed to deal with overmodulation region. V. STABILIZATION ANALYSIS OF POWER GENERATION OUTPUT CONTROL g f/v conv. s v g = v d gd uvw Interrupt cycle calc. Discontinuous PWM Fig. 7. Control block diagram of -phase inverter. Figure 8 shows the characteristics of jet engine thrust against the rotation speed. As shown Fig. 8, a thrust of 68. N is obtained at the rotation speed of 7 r/min. Under the atmospheric pressure, the atmospheric temperature, and the air density are constant, the thrust of the jet engine depends only on the rotational speed regardless of the output power. Since the thrust of the jet engine is proportional to the cube of the rotational speed, the thrust F obtained by the measured value is approximated by the cube of the rotational speed as follows; F k F (), where is rotation speed of the jet engine and k F is coefficient obtained from the measured value. Figure 9 shows the block diagram of the output power control system with a jet engine. In this system, the generator synchronous angular frequency g is produced P out Jet engine thrust F [N] 8 6 k M UX M UX v u, v, w v offset f c g mode V dc d u, v, w Carrier Modulation Gate signal S u, v, w 6 8 st I Rotation speed [r/min]( ) Fig. 8. Characteristics of jet engine thrust. Controller I K st P st I g s k vf r Plant Js eq.(8) P jet P th _ s P out Fig. 9. Block diagram of output power control system with jet engine. by the difference between the output power command P out and the output power detection value P out. Further, it is assumed that the response of the rotation speed control for the jet engine is sufficiently slower than that of the output power control. By ignoring the loss, the total power of the jet engine P jet is calculated by P P P (5), jet th out where P th is the thrust power of the jet engine. This thrust power is added to the shaft power P out that drives the propeller. This shaft power is determined by the flight speed and the thrust of the aircraft. However, if the aircraft is stationary as in the test, the shaft power cannot be calculated from the flight speed. In this case, the stationary shaft power P th_s is calculated by F Pth _ s 76 (6).. Substituting () into (6) and setting the coefficient as k th, the stationary shaft power [] is calculated by 76kF Pth _ s kth (7).. In order to analyze the stability of the control, the rotation angular velocity is linearized around the steadystate points. P k (8). th _ s th

5 8 Unstable region Inverter PWM rectifier Battery Imaginary part - =p.u. =.7p.u... =p.u. M G i u v uv Fig.. Configuration of experimental system. V out Output power [p.u.] Real part Fig.. Roots locus when the initial angular velocity is increased =.p.u. =.7p.u. P out.5 =p.u. =.p.u Time [s] Fig.. Step response of output power when output power command is changed from. p.u. to.5 p.u. te that o is the initial angular velocity at the steadystate point. Consequently, the transfer function from input to output of this control system is expressed by 9K pkvf kth Kir J Gs () (9), 9K pkvf kth s 9K pkvf kth s rj KirJ where K p is the proportional control gain, K i is the integral control gain, k vf is the voltage coefficient in the v/f control, J is the total inertia of the jet engine and generator, and r is the secondary winding resistance of the generator. Furthermore, K p and K i are expressed as functions of the damping coefficient and the response angular frequency n. nr J K p () 9k k ' vf th Ki () n te that ' is the initial angular velocity. This angular velocity should be set accordingly to the detection value of the angular velocity. However, the angular velocity detection is not employed in the test; therefore, this value is predetermined as following. Figure shows the roots locus when the initial angular velocity is increased. In this system, the TABLE III Specification of starter generator. Parameter Poles Rated rotary field speed Rated speed Rated voltage Rated current Rated power Rated torque Weight Diagram Full length Value 7 r/min 687 r/min V 5. A kw.6 N m. kg mm 9 mm powering operation is performed in the rotation speed range from.7 to. p.u. Therefore, the initial angular velocity setting value ' is.7 p.u. The damping coefficient is set to.7. The response angular frequency n is set; thus, the overshoot time is.5 seconds, which is / of the jet engine control period of.5 seconds. As shown in Fig., when the rotation speed is p.u., the control system is at the stability limit because the poles locate on the imaginary axis. The control system becomes stable because the poles move to the negative half plane when the rotation speed is larger than p.u. Figure shows the step response of output power when output power command is changed from. p.u. to.5 p.u. As shown in Fig., at a rotation speed of.7 p.u. and. p.u., the response is equal to or larger than the design response time. The response time is delayed and a large overshoot occurs in output power at the rotation speed of. p.u. and. p.u., which is the lowspeed range. However, such large overshoot does not occur since the power generation operation is performed only in the high-speed range in this system. VI. EXPERIMENTAL RESULTS A. Modulation method for even-order harmonic components suppression Figure shows the experimental system. Table III shows the specification of the starter generator. In this test, two motors shown in Table III are connected instead of the jet engine. In addition, a small capacity DC regulated power supply is connected to supply the excitation current at the time of starting since the starter generator is an induction generator. Figure shows a block diagram of the PWM converter. This control system is adjust the slip angle

6 frequency and control the DC-link voltage. In the high speed range, switching from the asynchronous PWM to the synchronous PWM. Furthermore, around the rated speed, discontinuous PWM is employed to deal with the overmodulation region. Figure shows the operation waveforms of the continuous PWM at frequency ratio of nine and the rotation speed of.8 p.u. The modulation index is.87, and both the conventional method and the proposed method control the output voltage to be constant at V. Figure 5 shows the harmonic analysis results of the generator current of the continuous PWM. As shown Fig. 5(b), the proposed method suppresses low even-order harmonic components, such as second, eighth, and tenth order, which are generated by the conventional method. V out ー V out PI Voltage control s j g f/v conv. s V g v u v V d = gd Interrupt cycle calc. uvw,, w Discontinuous PWM Fig.. Block diagram of PWM converter. f v offset c Also, the eighth harmonic component was reduced by 99.% compared to the conventional method. In addition, the generator current total harmonic distortion (THD) is reduced by 9.99% compared to the conventional method. Figure 6 shows the operation waveforms of the discontinuous PWM at a frequency ratio of nine and a rotation speed of. p.u. The modulation index is.8, and both the conventional method and the proposed method control the output voltage to be constant at V. Figure 7 shows the harmonic analysis results of the generator current of the discontinuous PWM. As Fig. 7(b) shown, the proposed method suppresses low evenorder harmonic components, such as second, eighth, and tenth order, which are generated by the conventional method. Also, the eighth harmonic component was reduced by 99.% compared to the conventional method. In addition, the generator current THD is reduced by 7.% compared to the conventional method. Therefore, this method is effective also in the discontinuous PWM. B. Control strategy for starter generator Figure 8 and Table IV shows the prototype of the jet generator and the specifications of the jet engine. As Input voltage vuv [5 V/div] Input voltage vuv [5 V/div] Input voltage vuv [5 V/div] Input voltage vuv [5 V/div] Generator current iu [ A/div] Generator current iu [ A/div] Output voltage Vout [5 V/div] Output voltage Vout [5 V/div] [ µs/div] [ µs/div] (a) Conventional method. (b) Proposed method. Fig.. Experimental results of continuous PWM. Generator current iu [ A/div] Generator current iu [ A/div] Output voltage Vout [5 V/div] Output voltage Vout [5 V/div] [ µs/div] [ µs/div] (a) Conventional method. (b) Proposed method. Fig. 6. Experimental results of discontinuous PWM. Generator current iu [p.u.] - - (9 Hz, 8.8 A) nd 7 th 8 th th th 7 th THD:86.5% 5 Frequency [Hz] (a) Conventional method. Generator current iu [p.u.] - - (9 Hz, 8.7 A) nd th 5 th 7th 8 th th th THD:7.% 5 Frequency [Hz] (a) Conventional method. Generator current iu [p.u.] - - (9 Hz, 7.8 A) 7 th th THD:77.% 7 th 9 th 5 Frequency [Hz] (b) Proposed method. Fig. 5. Frequency analysis results of continuous PWM. Generator current iu [p.u.] - - (9 Hz, 7.79 A) th THD:68.9% 7 th 7 th 5 Frequency [Hz] (b) Proposed method. Fig. 7. Frequency analysis results of discontinuous PWM.

7 shown in Fig. 8, the starter generator is connected to the jet engine without a speed reduction gear. Figure 9 shows the experimental waveforms of Run Mode at the rotation speeds of 7 r/min, where the output power to the battery is.98 kw. As shown in Fig. 9, the DC link voltage is regulated to V. Furthermore, the stable power generation operation is achieved since the battery current is constant at any rotation speeds. Figure shows the harmonic analysis results of the generator current in Fig. 9. As shown in Fig., evenorder low harmonic components are less than.% which is sufficiently smaller than the fundamental component. Figure shows the characteristics of the generator current THD against output power. As shown in Fig., the minimum current THD of 6.5% is achieved at the rotation speed of 7 r/min and the output power of.98 kw. This is because the fundamental component of the generator current increases as the output power increases. Figure shows the efficiency characteristics of the power converter. As shown in Fig., the maximum efficiency of 9.7% is achieved at the rotation speed of 7 r/min and the output power of.98 kw. Figure shows the temperature characteristics of the exhaust nozzle against rotation speed and the output power when the ambient temperature is 6 C. As shown in Fig., as the rotation speed of the jet engine increases, higher the output power is obtained at the same exhaust nozzle temperature. Further, this system has the highest efficiency when the exhaust nozzle temperature is around 8 C. Therefore, an output power command depends on angle frequency command g is as shown in Fig.. Figure shows the experimental results the jet generator operation with the exhaust nozzle temperature control. The exhaust nozzle temperature command is 8 C. The jet engine is accelerated to 6 r/min, 65 r/min, and 7 r/min in the run mode. Then the jet engine is decelerated to 6 r/min. As shown in Fig, even when the rotation speed accelerates or decelerates, the exhaust nozzle temperature converges to the command value. The exhaust nozzle temperature can be controlled within the maximum deviation of % compared to the command value in the steady state. Furthermore, the exhaust nozzle temperature at 7 r/min drops to 7 C because the inflow current of the battery is limited. In addition, the proposed control method achieves among the starting, the powering and the cooling operations. The transition without the deceleration is achieved by the synchronous frequency command limiter when the startup mode changes to the run mode. The output power gradually approaches zero after this transition, because the starter generator maintains the rotation speed until the output power of the jet engine becomes sufficiently high. The acceleration without overload is achieved in the startup mode by an output power limiter, which limits a command up to Generator current iu [%] Jet engine Starter generator Fig. 8. Prototype of jet generator. TABLE IV Specification of jet engine. Parameter Weight Diameter Full length Rated thrust Rated speed Value.9 kg mm 8 mm 65 N r/min DC voltage V dc [5 V/div] Battery current I bt [ A/div] Generator voltage v uv [5 V/div] Generator current i u [ A/div] [ µs/div] Fig. 9. Experimental waveforms of Run Mode. グラフタイトル THD:6.5% (.A Hz) Harmonic number Fig.. Harmonic components on the generator current. kw. Further, the starter generator decelerates by a free run when the operation mode transitions to the stop mode from the run mode. Then the inverter restarts at a rotation speed of r/min. The starter generator simultaneously performs the cooling operation. VII. CONCLUSION The control strategy for UAV with the jet engine were proposed in this paper. The stable transition without decelerating and overcurrent between the operation modes of the starter generator was achieved by the synchronous frequency command limit and the output power limiter. In addition, the even-order low harmonic

8 Generator current THD [%] Efficiency [%] r/min 6 r/min 7 r/min Output power [kw] Fig.. Characteristics of generator current total harmonics distortion r/min 6 r/min 7 r/min Output power [kw] Fig.. Characteristics of efficiency of power converter. components are suppressed by the modulation method using the estimated intersection phase for the synchronous PWM. The -kw prototype system achieved the maximum conversion efficiency of 9.7%, the minimum generator current THD of 6.5% at 7 r/min. Further, the exhaust nozzle temperature was controlled within the maximum deviation of % of the command value. ACKNOWLEDGMENT This paper is based on results obtained from a project subsidized by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. REFERRENCES [] A. C. Satici, H. Poonawala, M. W. Sppong: Robust Optical Control of Quadrotor UAVs, IEEE Access, vol., pp. 79-9,. [] N. Gageik, P. Benz, S. Montenegro: Obstacle Detection and Collision Avoidance for a UAV With Complementary Low-Cost Sensors, IEEE Access, vol., pp , 5 [] J. Shiau, D. Ma, P. Yang, G. Wang, J. Gong: Design of a Solar Power Management System for an Experimental UAV, IEEE Transactions on Aerospace and Electronic Systems, vol. 5,., pp. 5-6, 9 [] J. Shiau, D. Ma, P. Yang, G. Wang, J. Gong: Predictor-Based Control of a Class of Time-Delay Systems and Its Application to Quadrotors, IEEE Transactions on Industrial Electronics, vol. 6,., pp , 6 [5] M. Whittingham History, Evolution, and Future Status of Energy Storage, Proceedings of the IEEE, vol., pp. 58-5, [6] Small UAV turbojet engine developed in Japan. [7] S. Chuangpishit, A. Tabesh, Z. Shahrbabak M. Saeedifard: Topology Design for Collector Systems of Offshore Wind Farms With Pure DC Power Systems, IEEE Transactions on Industrial Electronics, vol. 6,., pp. -8, [8] Yosei Hirano, Takashi Yoshida, Kiyoshi Ohishi, Toshimasa Miyazaki, Yuki Yokokura, and Masataka Sato, Vibration Suppression Control Method for Trochoidal Reduction Gears Output power[kw] Ambient temperature : 6 C Exhaust nozzle temperature Output power command depend on g 85 C 8 C 75 C Rotation speed [r/min]( ) 7 C 65 C Fig.. Characteristics of exhaust nozzle temperature against rotation speed and output power, and output power command according to rotation speed. Output power [kw] Rotation speed [r/min]( ) Exhaust nozzle temperature[ C] Startup mode 8 C Run mode Stop mode Ambient temperature : C 5 Time[s] Fig.. Experimental results of operation of jet generator. under Load Conditions, IEEJ J. Industry Applications, vol.5, no., pp.67-75, 6 [9] Takashi Yoshioka, Thao Tran Phuong, Akinori Yabuki, Kiyoshi Ohishi, Toshimasa Miyazaki, and Yuki Yokokura, Highperformance Load Torque Compensation of Industrial Robot using Kalman-filter-based Instantaneous State Observer, IEEJ J. Industry Applications, vol.5, no., pp.67-75, 6. [] H. Kim, S. B. Lee, S. Park, S. H. Kia and G. A. Capolino, "Reliable Detection of Rotor Faults Under the Influence of Low- Frequency Load Torque Oscillations for Applications With Speed Reduction Couplings," IEEE Transactions on Industry Applications, vol. 5, no., pp. 6-68, 6 [] Toshiki Nakanishi, and Jun-ichi Itoh, Control Strategy for Modular Multilevel Converter based on Single-phase Power Factor Correction Converter, IEEJ J. Industry Applications, vol.6, no., pp.6-57, 7. [] E. Torenbeek: Synthesis of Subsonic Airplane Design, Kluwer Academic Publishers, pp. 97-,